A large number of scientific investigations are now performed on small animals, such as rats and mice. Typical topics of the research are pre-clinical testing of pharmacological substances and the investigation of molecular cascades in the living organisms by the fabrication of genetically modified animals (that have an altered expression of the molecules of interest).
Currently, more and more researchers use mice in the laboratories. There are several reasons for this. The first reason is purely economical: mice-keeping is much cheaper than keeping rats, cats, pigs, or primates. The second reason has a scientific background: the production of genetically modified mice is faster and cheaper than rats or other larger animals. Mice have short generation times, and tools exist for research purposes for mice. The exploitation of mice in such types of research has become a standard de facto.
Researchers are usually interested in the influence of a particular pharmacological agent or a genetic modification on the living organism. The primary indicators of such influence are changes in behavior. These changes are evidence of a biological activity of the substance in question or of the importance of a gene whose expression has been modified. Thus, behavioral testing of animals is an essential part of modern biological research.
Let us suppose that the behavior of a treated group of animals is changed. What can it tell us about the action of pharmacological substance? If a molecular cascade in which the investigated substance is involved is known, one often can derive a conclusion. However, in many cases there are only suggestions about the role of the new substance. In such cases, additional investigation is required. The next step in research is the detection of alterations in the organism. The study can be started practically at any level (biochemical, morphological, anatomical, physiological), but a particular choice is usually driven by a working hypothesis and technical capabilities.
The next logical step after a purely behavioral approach is a registration of alterations in physiological parameters of behaving animals. This step was often omitted in small animals because of technical limitations. Biological sensors and recording means were just too big to be attached and carried by small animals.
Animal behavior is controlled by the brain. The brain is a main target for many pharmacological substances. Because of this, it currently attracts more attention by researchers than any other organ. At present, the recording of electrical brain activity in mice, such as brain waves (electroencephalogram), neuronal action and field potentials, is conducted exclusively by means of wires attached to the head transmitting signals to stationary equipment. Such an arrangement is disturbing for the animals. It is also difficult to conduct such experiment during a prolonged period of time, as there is a risk that a connecting cable will be destroyed by the rodent. Such a cable also prevents the conducting of behavior-physiological experiments in some environments, for instance, in some mazes and big arenas.
Known radio telemetric systems eliminate cabling, but they need significant amount of energy for transmission of electrophysiological data, or require a receiving antenna of an unpractical size and geometry. The requirement of significant amount of energy limits the duration of an experiment and the relatively large size of the antenna prevents conducting of the experiment in certain environments. A typical Bluetooth short-range (10 m) radio transmitter consumes around 60 mA at its standard transmission rate of 721 kbps. A specially engineered biological transmitter can consume less power than a Bluetooth transmitter, but still requires substantial power, especially at high data transmission rates. The problem of high power consumption of radio transmitters has been partially solved by Yanagihara et al. (U.S. Pat. Pub. 2005/0085872 A1, April 2005). These authors suggest accumulating information temporarily in the transmitting device in local data storage, and then sending the stored data in a packet, while switching on the transmitter only during packet sending. This allows the researcher to exploit the transmitter in its optimal mode, i.e., to fully fill its bandwidth with the data. However, a higher transmission rate requires more power. Thus, this solution does not fully solve the problem of high power consumption of radio telemetry devices and techniques.
Another obstacle in using telemetric devices in such small animals as mice is the size of the transmitting module. The modules have become much smaller in the last several years. However, they are still too big to be carried at the head of a mouse, especially when considering that an additional signal-conditioning circuitry and a power source are needed. For instance, one of the smallest Bluetooth transmitters has dimensions 11.8×17.6×1.9 mm, including antenna (Mitsumi, WML-09). An example of transmitter attachment to the head of a rat is presented in the U.S. Pat. No. 4,852,573 (Kennedy, August 1989). Alternative methods of device attachment, such as attaching the device to the animal body or implanting it inside the animal, have severe disadvantages. Attaching a backpack disturbs the animal. Wires that go from the back pack to the head are in an extremely weak place and need protection. They can be damaged very quickly by the animal. Implantation of the device together with the wires under the skin or into the abdominal cavity (Bornhoft et al., WO 03/030581 A2, April 2003) is painful for the animal. This process generally requires at least five days of anesthetic treatment after such operation.
An additional disadvantage of telemetry is the interference of signals from several subjects in an experimental room. This is required, for instance, in sleep research. Signals from different devices attached to the several subjects should be transmitted in different frequency bands or should somehow be temporarily multiplexed. In a digital wireless network, one has to pay attention to not exceed the bandwidth of a receiver. This significantly increases the complexity of the entire experimental setup.
An alternative method is a data logging—when recorded variables are stored locally in the device and are downloaded at the end of a recording session by means of a standard wired interface. This method has become feasible due to profound progress in microelectronics during the last several years. Data logging has been implemented for patient monitoring in hospitals (Drakulic, U.S. Pat. No. 5,678,559, October 1997; Turner et al., U.S. Pat. Pub 2005/0131288 A1, June 2005) and also in animal research (Andrews, R. D., “Instrumentation for the remote monitoring of physiological and behavioral variables”, J. Appl. Physiol. 1998 November; 85(5):1974-81; Mavoori, J., et al., “An autonomous implantable computer for neural recording and stimulation in unrestrained primates”, J. Neurosci. Methods. 2005 October 15; 148(1):71-7; Vyssotski, A. L., et al., “Miniature neurologgers for flying pigeons: multichannel EEG and action and field potentials in combination with GPS recording”, J. Neurophysiol. 2006 February; 95(2):1263-73) (the Vyssotski article is hereby incorporated by reference in its entirety).
There are two main advantages of data logging in comparison with radio telemetry: low power consumption (even at high data rates) and very small size of the data storage. A typical commercially available 1.8V NAND Flash memory, which can be used in a transportable animal data logger, consumes 2 mA at a storing data rate of 60 kilobytes per second. Such dataflow is typically produced by four electrodes recording neuronal activity at a smallest allowed acquisition setup data rate of 10 ksps and 12-bit ADC resolution. Such current consumption is only 1/30 of the current consumption of a Bluetooth transmitter and, and at the same time, such data flow would practically completely occupy the Bluetooth's bandwidth. Thus, with the same battery, a data logger would run 30 times longer than a radio transmitter. A typical NAND Flash 256 MB memory chip in a FBGA cage has size of only 9.5×12.0×1.2 mm. The volume occupied by the memory module is almost 3 times smaller than a volume occupied by the smallest Bluetooth radio transmitter. The 256 MB of memory is enough to store 4 EEG channels during 5 days (100 samples per second/channel) or 4 channels of neuronal activity (neuronal spikes) during 1 h 10 min (10 kilo samples per second/channel). The duration of neuronal activity registration is enough for most short-term behavioral tests. Five days of non-stop EEG record can be useful in sleep research and also in monitoring disease development, for instance, under pharmacological treatment, and to test the efficacy of a pharmacological agent. If one needs a more prolonged registration period, the device can be exchanged with another one within several seconds. It does not significantly affect the continuity of the record.
In spite of very attractive features of data logging, it also suffers from a set of minor disadvantages. In spite of the extremely small size of the device, a mouse head is so small that the data logging device needs a special way of attachment to the head.
Another disadvantage is the absence of a synchronizing link between the data logging device and external devices. In many cases a researcher needs to know not only the state of the animal, but also the external conditions under which animal was in this state. If the animal receives some stimulation or demonstrates some behavior, one should precisely know which time points of the stored record in the data logging device corresponds to stimulus application times or behavioral episodes. Synchronization of the start of the recording with an external clock is not always sufficient, especially if some rapid neuronal responses are to be investigated. Mavoori et al. (2005) supplemented a data logger with an IR-link for this purpose. In his setup IR pulses were sent to the external equipment when neuronal spikes were detected. They were stored together with other behavioral information in an ordinary computer. The first disadvantage of such approach is its redundancy: some amount of data occurred was stored two times—in the logger and in the external device. The logger stores more data than it transmits via IR-link: the spike form occurred stored only in the logger. Such distributed accumulation of the experimental data increases the complexity of the subsequent analysis. The second disadvantage is that a wireless IR communication requires a direct line of sight between receiver and transmitter. This requirement is very difficult to fulfill with a free-moving animal. Thus, such type of communication is unreliable. The third disadvantage of this solution is that an IR transmitter also suffers from high power consumption as a radio transmitter, even if it transmits in short “bursts”. Peak current of a typical low-power IR transmitter is equal to 60 mA. As such, we conclude that such type of synchronizing communication cannot be used in a small mouse-fit data logger. A power-saving and economical way of synchronizing of the data logger with external events is an aspect of the current invention.